EM 1110-1-1004 1 June 2002

US Army Corps of Engineers

ENGINEERING AND DESIGN

Geodetic and Control Surveying

ENGINEER MANUAL

CECW-EE Manual No. 1110-1-1004

DEPARTMENT OF THE ARMY US Army Corps of Engineers Washington, DC 20314-1000

EM 1110-1-1004

1 June 2002 Engineering and Design GEODETIC AND CONTROL SURVEYINGTable of Contents

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Chapter 1 Introduction Purpose............................................................................................................1-1 Applicability.....................................................................................................1-2 Distribution ......................................................................................................1-3 References .......................................................................................................1-4 Background......................................................................................................1-5 Scope of Manual...............................................................................................1-6 Life Cycle Project Management.........................................................................1-7 Metrics ............................................................................................................1-8 Trade Name Exclusions.....................................................................................1-9 Abbreviations and Terms ..................................................................................1-10 Mandatory Requirements ..................................................................................1-11 Proponency......................................................................................................1-12 Chapter 2 Control Surveying Applications General............................................................................................................2-1 Project Control Densification.............................................................................2-2 Geodetic Control Densification..........................................................................2-3 Vertical Control Densification ...........................................................................2-4 Structural Deformation Studies..........................................................................2-5 Photogrammetry...............................................................................................2-6 Dynamic Positioning and Navigation .................................................................2-7 GIS Integration.................................................................................................2-8

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Chapter 3 Standards and Specifications for Control Surveying General............................................................................................................3-1 Accuracy .........................................................................................................3-2 General Procedural Standards and Specifications ................................................3-3 Construction Surveys ........................................................................................3-4 Cadastral and Real Estate Surveys .....................................................................3-5 Geodetic Control Surveys..................................................................................3-6 Topographic Site Plan Mapping Surveys ............................................................3-7 Structural Deformation Surveys .........................................................................3-8 Photogrammetric Mapping Control Surveys .......................................................3-9 Hydrographic Surveys.......................................................................................3-10 GIS Surveys .....................................................................................................3-11 Mandatory Requirements ..................................................................................3-12 Chapter 4 Reference Systems and Datum Transformations Reference Systems ...........................................................................................4-1 Geodetic Coordinates........................................................................................4-2 State Plane Coordinate System ..........................................................................4-3 Universal Transverse Mercator Coordinate System.............................................4-4 Datum Transformations.....................................................................................4-5 Horizontal Datum Transformations ....................................................................4-6 Horizontal Transition Plan.................................................................................4-7 Vertical Datums ...............................................................................................4-8 Vertical Datum Transformations ........................................................................4-9 Vertical Transition Plan.....................................................................................4-10 Mandatory Requirements ..................................................................................4-11 Chapter 5 Horizontal Control Survey Techniques Introduction......................................................................................................5-1 Secondary Horizontal Control............................................................................5-2 Traverse Survey Standards ................................................................................5-3 Traverse Survey Guidelines...............................................................................5-4 Traverse Classifications ....................................................................................5-5 Triangulation and Trilateration ..........................................................................5-6 Bearing and Azimuth Determination..................................................................5-7 Mandatory Requirements ..................................................................................5-8 Chapter 6 Vertical Control Survey Techniques General............................................................................................................6-1 Second Order Leveling......................................................................................6-2 Third Order Leveling ........................................................................................6-3 Mandatory Requirements ..................................................................................6-4

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Chapter 7 Miscellaneous Field Notekeeping and Procedural Requirements Field Notes.......................................................................................................7-1 Horizontal Control Survey Field Notes...............................................................7-2 Vertical Control Survey Field Notes ..................................................................7-3 Rights-of-Entry.................................................................................................7-4 Mandatory Requirements ..................................................................................7-5

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Appendix A References Appendix B CORPSCON Technical Documentation and Operating Instructions Appendix C Development and Implementation of NAVD 88 Appendix D Requirements and Procedures for Referencing Coastal Navigation Projects to Mean Lower Low Water (MLLW) Datum Glossary

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Chapter 1 Introduction1-1. Purpose This manual provides technical specifications and procedural guidance for control and geodetic surveying. It is intended for use by engineering, topographic, and construction surveyors performing control surveys for civil works, military construction, and environmental restoration projects. Procedural and quality control standards are defined to establish uniformity in control survey performance and contract administration. 1-2. Applicability This manual applies to all USACE commands having responsibility for the planning, engineering and design, operations, maintenance, construction, and related real estate and regulatory functions of civil works, military construction, and environmental restoration projects. It applies to control surveys performed by both hired-labor forces and contracted survey forces. 1-3. Distribution This publication is approved for public release; distribution is unlimited. 1-4. References Referenced USACE publications are listed in Appendix A. Where applicable, bibliographic information is listed within each chapter or appendix. 1-5. Background A geodetic control survey consists of establishing the horizontal and vertical positions of points for the control of a project or installation site, map, GIS, or study area. These surveys establish threedimensional point positions of fixed monuments, which then can provide the primary reference for subsequent engineering and construction projects. These control points also provide the basic framework from which detailed site plan topographic mapping, boundary demarcation, and construction alignment work can be performed. Precisely controlled monuments are also established to position marine construction vessels supporting the Corps navigation mission--e.g., the continuous positioning of dredges and survey boats. Geodetic control survey techniques are also used to effectively and efficiently monitor and evaluate external deformations in large structures, such as locks and dams. 1-6. Scope of Manual This manual covers the use of engineering surveying techniques for establishing and/or extending project construction control. Accuracy requirements, standards, measurement procedures, calibrations, horizontal and vertical datum transformations, data reduction and adjustment methods, and engineering surveying techniques are outlined. The primary focus of this manual is on conventional (i.e., non-GPS) horizontal and vertical survey techniques using traditional ground survey instruments--transits, theodolites, levels, electronic total stations, etc. Typically, conventional survey techniques include traverse, triangulation, trilateration, and differential leveling.

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a. The manual is intended to be a reference guide for control surveying, whether performed by in-house hired-labor forces, contracted forces, or combinations thereof. General planning criteria, field and office execution procedures, project datum requirements, and required accuracy specifications for performing engineering surveys are provided. Accuracy specifications, procedural criteria, and quality control requirements contained in this manual should be directly referenced in the scopes of work for Architect-Engineer (A-E) survey services or other third-party survey services. This ensures that standardized procedures are followed by both hired-labor and contract service sources. b. The survey performance criteria given in this manual are not intended to meet the Federal Geodetic Control Subcommittee (FGCS) standards required for densifying the National Geodetic Reference System (NGRS). However, the methods and procedures given in this manual will yield results equal to or exceeding FGCS Second Order relative accuracy criteria. Second Order accuracy is generally considered sufficient for most USACE engineering and construction work. c. This manual does not cover the concepts of using differential GPS for performing precise geodetic control surveys. For further specific guidance on all aspects of GPS surveying, the user should consult EM 1110-1-1003, NAVSTAR GPS Surveying. d. This manual should be used in conjunction with other USACE surveying and mapping engineering manuals that refer to it for guidance on datums and datum transformation procedures. These procedures are covered in Chapter 4 (Reference Systems and Datum Transformations) and in Appendices B, C, and D. e. This manual was initially developed as part of the 31 October 1994 version of EM 1110-1-1004, "Deformation and Control Surveying." During the current update, structural deformation surveying portions of the 1994 manual were removed and incorporated into a separate technical manual. The current version of EM 1110-1-1004 was then re-titled as "Geodetic and Control Surveying" to reflect the revised scope. 1-7. Life Cycle Project Management Project control surveys may be required through the entire life cycle of a project, spanning decades in many cases. During the early planning phases of a project, a comprehensive control plan should be developed which considers survey requirements over a project's life cycle, with a goal of eliminating duplicate or redundant surveys to the maximum extent possible. 1-8. Metrics Both English and metric units are used in this manual. Metric units are commonly used in precise surveying applications, including the horizontal and vertical survey work covered in this manual. Control survey measurements are usually recorded and reported in metric units. In all cases, the use of either metric or English units shall follow local engineering and construction practices. 1-9. Trade Name Exclusions The citation or illustration in this manual of trade names of commercially available survey products, including other auxiliary surveying equipment, instrumentation, and adjustment software, does not constitute official endorsement or approval of the use of such products.

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1-10. Abbreviations and Terms Abbreviations used in this manual are defined in the Glossary at the end of this manual. Commonly used engineering surveying terms are also explained in the Glossary. 1-11. Mandatory Requirements ER 1110-2-1150 (Engineering and Design for Civil Works Projects) prescribes that mandatory requirements be identified in engineer manuals. Mandatory requirements in this manual are summarized at the end of each chapter. Mandatory accuracy standards, quality control, and quality assurance criteria are normally summarized in tables within each chapter. The mandatory criteria contained in this manual are based on the following considerations: (1) project safety considerations, (2) overall project function, (3) previous Corps experience and practice has demonstrated the criteria are critical, (4) Corps-wide geospatial data standardization requirements, (5) adverse economic impacts if criteria are not followed, and (6) HQUSACE commitments to Federal and industry standards. 1-12. Proponency The HQUSACE proponent for this manual is the Engineering and Construction Division, Directorate of Civil Works (CECW-EE). Technical development and compilation of the manual was coordinated by the US Army Topographic Engineering Center (CEERD-TS-G). Comments, recommended changes, or waivers to this manual should be forwarded through MSC to HQUSACE (ATTN: CECW-EE).

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Chapter 2 Control Surveying Applications2-1. General Control surveys are used to support a variety of USACE project applications. These include project boundary control densification, structural deformation studies, photogrammetric mapping, dynamic positioning and navigation for hydrographic survey vessels and dredges, hydraulic study/survey location, river/floodplain cross-section location, core drilling location, environmental studies, levee overbank surveys, levee profiling, levee grading and revetment placement, disposal area construction, grade control, real property surveys, and regulatory enforcement actions. Some of these applications are described below. 2-2. Project Control Densification a. Conventional surveying. Conventional geodetic control surveys are those performed using traditional precise surveying techniques and instruments--i.e., theodolites, total stations, and levels. Conventional control surveys can be used to economically and accurately establish or densify project control in a timely fashion. Quality control statistics and redundant measurements in networks established by these methods help to ensure reliable results. However, conventional survey methods do have the requirement for intervisibility between adjacent stations. b. GPS surveying. GPS satellite surveying techniques can often be used to establish or densify project control more efficiently (and accurately) than conventional control surveying techniques-especially over large projects. As with conventional methods, quality control statistics and redundant measurements in GPS networks help to ensure reliable results. Field operations to perform a GPS survey are relatively easy and can generally be performed by one person per receiver, with two or more receivers required to transfer control. GPS does not require intervisibility between adjacent stations. However, GPS must have visibility of at least four satellites during surveying. This requirement may make GPS inappropriate in areas of dense vegetation. For GPS control survey techniques refer to EM 1110-1-1003, NAVSTAR GPS Surveying. 2-3. Geodetic Control Densification Conventional control and GPS surveying methods can be used for wide-area, high-order geodetic control densification. First-, Second- or Third-Order work can be achieved using conventional or GPS surveying techniques. GPS techniques are now generally used for most horizontal control surveys performed for mapping frameworks. Conventional instruments and procedures are generally preferred for site plan topographic mapping and critical construction control. Topographic mapping procedures used in detailed site plan surveys are contained in EM 1110-1-1005, Topographic Surveying. 2-4. Vertical Control Densification Conventional leveling methods are used to determine orthometric height elevations of benchmarks established for vertical control densification. The setup and operation for conventional control surveying for vertical control densification offers economies of scale in the same manner as that offered by the setup for horizontal project control densification--i.e., smaller projects require less setups, while larger projects require more. For large mapping projects, differential GPS may prove more cost effective for densifying vertical control. However, for small project sites or construction projects, conventional spirit leveling is generally preferred.2-1

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2-5. Structural Deformation Studies a. Conventional control surveying can be used to monitor the motion of points on a structure relative to stable monuments. This is usually done using an Electronic Distance Measuring (EDM) instrument located on various stable reference monuments away from the structure, and measuring precise distances to calibrated reflectors positioned at selected points on the structure. When only distances are measured, trilateration techniques may be employed to compute absolute movements. If angular observations are added, such as with a theodolite or electronic total station, then triangulation methods may be added to a position solution. These precise techniques can provide a direct measure of the displacement of a structure as a function of time. If procedures are strictly adhered to, it is possible to achieve a +0.5 mm + 4 ppm (4 mm/km) baseline accuracy using conventional surveying instruments. Personnel requirements generally are two, once the initial test network of reference and object points are set up--one person to monitor the EDM or total station and another to aid in reflector placement. b. GPS can also be used to monitor the motion of points on a structure relative to stable monuments. With GPS, an array of antennae are positioned at selected points on the structure and on remote stable monuments--as opposed to using reflectors and EDMs as previously described. The baselines between the antennae are formulated to monitor differential movement. The relative precision of the measurements is on the order of +5 mm over distances averaging between 5 and 10 km, and near the 1-mm level for short baselines. GPS observations can be determined continuously 24 hours a day. Once a deformation monitoring system has been set up using GPS, it can be operated unattended and is relatively easy to maintain. 2-6. Photogrammetry Geodetic control surveys are used in the support of photogrammetric mapping applications. These control surveys are performed to provide rigid horizontal and vertical alignment of the photographs. Since photogrammetric mapping projects typically are large in extent, GPS methods have largely replaced conventional control survey techniques. In many cases, photogrammetric mapping control surveys have been largely eliminated through the use of differential GPS-controlled airborne cameras. More specific guidance on the use of control surveying in support of photogrammetry is included in EM 1110-1-1000, Photogrammetric Mapping. 2-7. Dynamic Positioning and Navigation a. Conventional control surveying can be used to establish the primary project control for the dynamic positioning and navigation of construction and surveying platforms used for design, construction, and environmental regulatory efforts. These efforts include dredge control systems, site investigation studies/surveys, horizontal and vertical construction placement, hydraulic studies, or any other waterborne activity requiring two- or three-dimensional control. Second Order or Third Order leveling is required for these efforts. b. GPS has reduced (or even eliminated in many cases) the time and effort required to establish control for dynamic positioning and navigation systems. In addition to this capability, GPS equipment can provide dynamic, real-time GPS code and carrier phase positioning of construction and surveying platforms. GPS code phase differential techniques can provide real-time meter-level horizontal positioning and navigation, while GPS carrier phase differential techniques can provide real-time, centimeter-level, three-dimensional positioning and navigation. These GPS methods can be used for any type of construction or survey platform (e.g., dredges, graders, survey vessels, etc.). More specific guidance on the use of GPS for dynamic positioning and navigation is included in EM 1110-2-1003, Hydrographic Surveying.2-2

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2-8. GIS Integration A Geographic Information System (GIS) can be used to correlate and store diverse information on natural or man-made characteristics of geographic positions. To effectively establish and use a GIS, it must be based on accurate geographic coordinates. A GIS with an accurate foundation of geographic coordinates enables the user to readily exchange information between databases. Conventional control surveying and GPS surveying can be used to establish the geographic coordinates used as the foundation for a GIS. Refer to EM 1110-1-2909, Geospatial Data and Systems, for detailed guidance on GIS development.

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Chapter 3 Standards and Specifications for Control Surveying 3-1. General This chapter details standards and specifications for control surveying and provides guidance on how to achieve them. 3-2. Accuracy The accuracy of control surveying measurements should be consistent with the purpose of the survey. When evaluating the techniques to be used and accuracies desired, the surveyor must evaluate the limits of the errors of the equipment involved, the procedures to be followed, and the error propagation. These evaluations should be firmly based on past experience or written guidance. It is important to remember in this evaluation that the best survey is the one that provides the data at the required accuracy levels, without wasting manpower, time, and money. a. Survey accuracy standards prescribed in this section relate to the relative accuracy derived from a particular survey. This relative accuracy (or precision) is estimated by internal closure checks of the survey run through the local project, map, or construction site. Relative survey accuracy estimates are traditionally expressed as ratios of the misclosure to the total length of the survey (e.g., 1:10,000). Relative survey accuracies are different than map accuracies, which are expressed in terms of limiting positional error. Since map compilation is dependent on survey control, map accuracies will ultimately hinge on the adequacy and accuracy of the base survey used to control the map. b. Tables 3-1 and 3-2 detail the basic minimum criteria required for planning, performing, and evaluating the adequacy of control surveys. c. These criteria apply to all conventional control surveying as well as GPS surveying activities-the intended accuracy is independent of the survey equipment employed. d. Many of the criteria shown in the tables are developed from FGCS standards for performing conventional control surveys and GPS surveys. The criteria listed in the tables have been modified to provide more practical standards for engineering and survey densification. FGCS Standards and Specifications for Geodetic Control Networks (FGCS 1984) covers all aspects of performing conventional control surveys for high-precision geodetic network densification purposes, while FGCS GPS survey standards (FGCS 1988) covers the use of GPS surveys for the same application. The application of some FGDC standards and techniques are not always practical for typical civil works, military construction, and environmental restoration activities where lower precision control is acceptable. e. If a primary function of a survey is to support NGRS densification, then specifications listed in the FGCS publications (FGCS 1984 and FGCS 1988) should be followed in lieu of those in Tables 3-1 and 3-2.

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Table 3-1. USACE Point Closure Standards for Horizontal Control Surveys (ratio) USACE Classification Second Order Class I Second Order Class II Third Order Class I Third Order Class II Fourth Order Point Closure Standard 1:50,000 1:20,000 1:10,000 1: 5,000 1:2,500 - 1:20,000

Table 3-2. USACE Point Closure Standards for Vertical Control Surveys (in feet) USACE Classification Second Order Class I Second Order Class II Third Order Fourth Order Point Closure Standard 0.025*sqrt M 0.035*sqrt M 0.050*sqrt M 0.100*sqrt M

where sqrt M = square root of distance M in miles

(1) Survey classification. A survey shall be classified based on its horizontal point closure ratio, as indicated in Table 3-1, or the vertical elevation difference closure standard given in Table 3-2. (2) Horizontal control standards. The horizontal point closure is determined by dividing the linear distance misclosure of the survey into the overall circuit length of a traverse, loop, or network line/circuit. When independent directions or angles are observed, as on a conventional survey (i.e., traverse, trilateration, or triangulation), these angular misclosures may optionally be distributed before assessing positional misclosure. In cases where GPS vectors are measured in geocentric coordinates, then the three-dimensional positional misclosure is assessed. (a) Approximate surveying. Approximate surveying work should be classified based on the surveys estimated or observed positional errors. This would include absolute GPS and some differential GPS techniques with positional accuracies ranging from 10 to 150 feet (95% RMS). There is no order classification for such approximate work. (b) Higher order survey. Requirements for relative line accuracies exceeding 1:50,000 are rare for most USACE applications. Surveys requiring accuracies of First Order (1:100,000) or better should be performed using FGCS standards and specifications, and must be adjusted by the NGS. (c) Construction layout or grade control. This classification is analogous to traditional Fourth Order work. It is intended to cover temporary control used for alignment, grading, and measurement of various types of construction, and some local site plan topographic mapping or photo mapping control work. Accuracy standards will vary with the type of construction. Lower accuracies (1:2,500 - 1:5,000) are acceptable for earthwork, embankment, beach fill, and levee alignment stakeout and grading, and some site plan, curb and gutter, utility building foundation, sidewalk, and small roadway stakeout. Moderate accuracies (1:5,000) are used in most pipeline, sewer, culvert, catch basin, and manhole stakeout and for general residential building foundation and footing construction, major highway3-2

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pavement, and concrete runway stakeout work. Somewhat higher accuracies (1:10,000 -1: 20,000) are used for aligning longer bridge spans, tunnels, and large commercial structures. For extensive bridge or tunnel projects, 1:50,000 or even 1:100,000 relative accuracy alignment work may be required. Vertical grade is usually observed to the nearest 0.01 foot for most construction work, although 0.1-foot accuracy is sufficient for riprap placement, grading, and small diameter pipe placement. Construction control points are typically marked by semi-permanent or temporary monuments (e.g., plastic hubs, P-K nails, wooden grade stakes). Control may be established by short, non-redundant spur shots, using total stations or GPS, or by single traverse runs between two existing permanent control points. Positional accuracy will be commensurate with, and relative to, that of the existing point(s) from which the new point is established. (3) Vertical control standards. The vertical accuracy of a survey is determined by the elevation misclosure within a level section or level loop. For conventional differential or trigonometric leveling, section or loop misclosures (in feet) shall not exceed the limits shown in Table 3-2, where the line or circuit length (M) is measured in miles. Fourth Order accuracies are intended for construction layout grading work. Procedural specifications or restrictions pertaining to vertical control surveying methods or equipment should not be overly restrictive. (4) Contract compliance with USACE survey standards. Contract compliance assessment shall be based on the prescribed point closure standards of internal loops, not on closure with external networks of unknown accuracy. In cases where internal loops are not observed, then assessment must be based on external closures. Specified closure accuracy standards shall not be specified that exceed those required for the project, regardless of the accuracy capabilities of the survey equipment. 3-3. General Procedural Standards and Specifications a. Most survey applications in typical civil works and military arenas can be satisfied with a Second- or Third Order level of accuracy. Higher levels of accuracy are required for the densification of high-precision geodetic networks and some forms of deformation monitoring. b . Since most modern survey equipment (e.g., GPS or electronic total stations) are capable of achieving far higher accuracies than those required for engineering, construction, and mapping, only generalized field survey specifications are necessary for most USACE work. The following paragraphs outline some of the more critical specifications that relate to the USACE horizontal and vertical standards. Additional guidance for performing control surveying is found in subsequent chapters, as well as in some of the technical manuals listed in Appendix A. (1) Survey instrumentation criteria. USACE Commands shall minimize the use of rigid requirements for particular surveying equipment or instruments used by professional surveying contractors. In some instances, contract technical specifications may prescribe a general type of instrument system be employed (e.g., total station, GPS, spirit level), along with any unique operating or calibration requirements. (2) Survey geometry and field observing criteria. In lieu of providing detailed government procedural specifications, professional contractors may be presumed capable of performing surveys in accordance with accepted industry standards and practices. Any geometrical form of survey network may be formed: traverses, loops, networks, and cross-links within networks. Traverses should generally be closed back (or looped) to the same point, to allow an assessment of the internal misclosure accuracies. Survey alignment, orientation, and observing criteria should not be rigidly specified; however, guidance regarding limits on numbers of traverse stations, minimum traverse course lengths, auxiliary connections, etc. are provided in the subsequent chapters of this EM, as well in other EMs listed in Appendix A.3-3

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(3) Connections to existing control. USACE surveys should be connected to existing local control or project control monuments/benchmarks. These existing points may be those of any Federal (including USACE project control), state, local, or private agency. Ties to local USACE project control and boundary monuments are absolutely essential and critical to design, construction, and real estate. In order to minimize scale or orientation errors, at least two existing monuments should be connected, if practicable. However, survey quality control accuracy assessments (Tables 3-1 and 3-2) shall be based on internal traverse or level line closures--not on external closures between or with existing monuments or benchmarks. Accuracy assessments based on external closures typically require a knowledge of the statistical variances in the fixed network. (4) Connections to NGRS control. The NGRS pertains to geodetic control monuments with coordinate or elevation data published by the NGS. It is recommended that USACE surveys be connected with one or more stations on the NGRS when practicable and feasible. Connections with the NGRS shall be subordinate to the requirements for connections with local/project control. Connections with local/project control that has previously been connected to the NGRS are adequate in most cases. (5) Survey datums. A variety of survey datums and references are used throughout USACE projects. It is recommended that horizontal surveys be referenced to the North American Datum of 1983 (NAD 83) or, if NAD 83 is unavailable, to the NAD 27 system, with coordinates referred to the local State Plane Coordinate System (SPCS) for the area. The NAD 83 is the preferred reference datum. The Universal Transverse Mercator (UTM) grid system may be used for military operational or tactical uses, in OCONUS locales without a local coordinate system, or on some civil projects crossing multiple SPCS zones. Vertical control should be referenced to either the National Geodetic Vertical Datum, 1929 Adjustment (NGVD 29) or the North American Vertical Datum, 1988 Adjustment (NAVD 88). Independent survey datums and reference systems shall be avoided unless required by local code, statute, or practice. This includes local tangent grid systems, state High Accuracy Reference Networks (HARN), and unreferenced construction baseline station-offset control. (6) Spur points. Spur points (open-ended traverses or level lines) should be avoided to the maximum extent practicable. In many cases, it is acceptable survey procedure for temporary Fourth Order construction control, provided adequate blunder detection is taken. Kinematic differential GPS (DGPS code or carrier phase tracking) surveys are effectively spur point surveys with relative accuracies well in excess of Third Order standards. These DGPS kinematic spur techniques may be acceptable procedures for most control surveying in the future, provided blunder protection procedures are developed. Refer to EM 1110-1-1003 for further GPS guidance. (7) Survey adjustments. The standard adjustment method in USACE will be either the Compass Rule or Least Squares. Technical (and contractual) compliance with accuracy standards prescribed in Tables 3-1 and 3-2 will be based on the internal point misclosures. Propagated relative distance/line accuracy statistics used by FGCS that result from unconstrained (minimally constrained) least squares adjustment error propagation statistics may be assumed comparable to relative misclosure accuracy estimates for survey quality control assessment. Surveys may be adjusted (i.e., constrained) to existing control without regard to the variances in the existing network adjustment. Exceptions to this requirement are surveys performed to FGCS standards and specifications that are adjusted by NGS. (8) Data recording and archiving. Field survey data may be recorded either manually or electronically. Manual recordation should follow industry practice, using formats outlined in various technical manuals (Appendix A). Refer to EM 1110-1-1005, Topographic Surveying, for electronic survey data collection standards.

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3-4. Construction Surveys In-house and contracted construction surveys generally will be performed to meet Third Order-Class II (1:5,000) accuracy. Some stakeout work for earthwork clearing and grading, and other purposes, may need only be performed to meet Fourth Order accuracy requirements. Other stakeout work, such as tunnel or bridge pier alignment, may require Second Order or higher accuracy criteria. Construction survey procedural specifications should follow recognized industry practices. 3-5. Cadastral and Real Estate Surveys Many state codes, rules, or statutes prescribe minimum technical standards for surveying and mapping. Generally, most state accuracy standards for real property surveys parallel USACE Third Order point closure standards--usually ranging between 1:5,000 and 1:10,000. USACE and its contractors shall follow applicable state minimum technical standards for real property surveys involving the determination of the perimeters of a parcel or tract of land by establishing or reestablishing corners, monuments, and boundary lines, for the purpose of describing, locating fixed improvements, or platting or dividing parcels. Although state minimum standards relate primarily to accuracies of land and boundary surveys, other types of survey work may also be covered in some areas. See also ER 405-1-12, Real Estate Handbook. Reference also the standards and specifications prescribed in the Manual of Instruction for the Survey of the Public Lands of the United States (US Bureau of Land Management 1947) for cadastral surveys, or surveys of private lands abutting or adjoining Government lands. 3-6. Geodetic Control Surveys a. Geodetic control surveys are usually performed for the purpose of establishing a basic framework to be included in the national geodetic reference network, or NGRS. These geodetic survey functions are distinct from the survey procedures and standards defined in this EM which are intended to support USACE engineering, construction, mapping, and Geographic Information System (GIS) activities. b. Geodetic control surveys of permanently monumented control points that are to be incorporated in the NGRS must be performed to far more rigorous standards and specifications than are surveys used for general engineering, construction, mapping, or cadastral purposes. When a project requires NGRS densification, or such densification is a desirable by-product and is economically justified, USACE Commands should conform to FGCS survey standards and specifications, and other criteria prescribed under Office of Management and Budget (OMB) Circular A-16 (OMB 1990). This includes related automated data recording, submittal, project review, and adjustment requirements mandated by FGCS and NGS. Details outlining the proposed use of FGCS standards and specifications in lieu of USACE standards, including specific requirements for connections to the NGRS, shall be included in the descriptions of survey and mapping activities contained in project authorization documents. c. Geodetic control surveys intended for support to and inclusion in the NGRS must be done in accordance with the following FGCS publications: (1) Standards and Specifications for Geodetic Control Networks (FGCS 1984). (2) Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Techniques (FGCS 1988). (3) Input Formats and Specifications of the National Geodetic Data Base (also termed the Bluebook) (FGCS 1980).3-5

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(4) Guidelines for Submitting GPS Relative Positioning Data to the National Geodetic Survey (NGS 1988). A survey performed to FGCS accuracy standards and specifications cannot be definitively classified or certified until the NGS has performed a variance analysis of the survey relative to the existing NGRS. This analysis and certification cannot be performed by USACE Commands-- only the NGS can perform this function. It is estimated that performing surveys to meet these FGCS standards, specifications, and archiving criteria can add between 25 and 50 percent to the surveying costs of a project. Therefore, sound judgment must be exercised on each project when determining the practicability of doing survey work that, in addition to meeting the needs of the project, can be used for support to and inclusion in the NGRS. 3-7. Topographic Site Plan Mapping Surveys Control surveys from which site plan mapping is densified (using plane tables, electronic total stations, or GPS) are normally established to USACE Third Order standards. Follow guidance in EM 1110-1-1005, Topographic Surveying. 3-8. Structural Deformation Surveys Structural deformation surveys are performed in compliance with the requirements in ER 1110-2-100, often termed PICES surveys. PICES surveys require high line vector and/or positional accuracies to monitor the relative movement of monoliths, walls, embankments, etc. PICES survey accuracy standards vary with the type of construction, structural stability, failure probability, and impact, etc. In general, horizontal and vertical deformation monitoring survey procedures are performed relative to a control network established for the structure. Ties to the NGRS or NGVD 29 are not necessary other than for general reference; and then only an USACE Third Order connection is needed. FGCS geodetic relative accuracy standards are not applicable to these localized movement surveys. Other deformation survey and instrumentation specifications and procedures for earth and rock fill dams and concrete structures are in EM 1110-2-1908 and EM 1110-2-4300. 3-9. Photogrammetric Mapping Control Surveys Control surveys required for controlling photogrammetric mapping products will normally be performed to USACE Third Order standards. Occasionally USACE Second Order standards will be required for extensive aerotriangulation work. Reference EM 1110-1-1000, Photogrammetric Mapping, for detailed photogrammetric mapping standards and specifications. 3-10. Hydrographic Surveys Control points for USACE hydrographic surveys generally are set to Third Order horizontal and vertical accuracy. Exceptions are noted in EM 1110-2-1003, Hydrographic Surveying. Hydrographic depth sounding accuracies are based on the linear and radial error measures described in EM 1110-2-1003, as well as hydrographic survey procedural specifications.

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3-11. GIS Surveys GIS raster or vector features can be scaled or digitized from any existing map. Typically a standard USGS 1:24,000 quadrangle map is adequate given the accuracies needed between GIS data features, elements, or classifications. Relative or absolute GPS (i.e., 10- to 30-foot precision) survey techniques may be adequate to tie GIS features where no maps exist. Second- or Third Order control networks are generally adequate for all subsequent engineering, construction, real estate, GIS, and/or Automated Mapping/Facilities Management (AM/FM) control. 3-12. Mandatory Requirements The accuracy standards in Tables 3-1 and 3-2 are mandatory.

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Chapter 4 Reference Systems and Datum Transformations4-1. Reference Systems a. General. The discipline of surveying consists of locating points of interest on the surface of the earth. The positions of points of interest are defined by coordinate values that are referenced to a predefined mathematical surface. In geodetic surveying, this mathematical surface is called a datum, and the position of a point with respect to the datum is defined by its coordinates. The reference surface for a system of control points is specified by its position with respect to the earth and its size and shape. A datum is a coordinate surface used as reference figure for positioning control points. Control points are points with known relative positions tied together in a network. Densification of the network of control points refers to adding more control points to the network and increasing its scope. Both horizontal and vertical datums are commonly used in surveying and mapping to reference coordinates of points in a network. Reference systems can be based on the geoid, ellipsoid, or a plane. The physical earths gravity force can be modeled to create a positioning reference frame that rotates with the earth. The geoid is such a surface (an equipotential surface of the earths gravity field) that best approximates Mean Sea Level (MSL)--see Figure 4-1. The orientation of this surface at a given point on geoid is defined by the plumb line. The plumb line is oriented tangent to the local gravity vector. Surveying instruments can be readily oriented with respect to the gravity field because its physical forces can be sensed with simple mechanical devices. A mean gravity field can be used as a reference surface to represent the actual earths gravity field. Such a reference surface is developed from an ellipsoid of revolution that best approximates the geoid. An ellipsoid of revolution provides a well-defined mathematical surface to calculate geodetic distances, azimuths, and coordinates. The major semi-axis (a ) and minor semi-axis (b ) are the parameters used to determine the ellipsoid size and shape. The shape of a reference ellipsoid also can be described by either its flattening (f) or its eccentricity (e). Flattening: Eccentricity: f = (a-b) / a e = [ sqrt ( a 2 - b 2 ) ] / a

Earths surface Geoid EllipsoidGeoid Undulation Deflection of the Vertical

Perpendicular to Ellipsoid

Perpendicular to Geoid (Plumbline)

Figure 4-1. The relationship between the ellipsoid, geoid, and the physical surface of the earth

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4-2. Geodetic Coordinates a. General. A coordinate system is defined by the location of the origin, orientation of its axes, and the parameters (coordinate components) which define the position of a point within the coordinate system. Terrestrial coordinate systems are widely used to define the position of points on the terrain because they are fixed to the earth and rotate with it. The origin of terrestrial systems can be specified as either geocentric (origin at the center of the earth) or topocentric (origin at a point on the surface of the earth). The orientation of terrestrial coordinate systems is described with respect to its poles, planes, and axes. The primary pole is the axis of symmetry of the coordinate system, usually parallel to the rotation axis of the earth, and coincident with the minor semi-axis of the reference ellipsoid. The reference planes that are perpendicular to the primary pole are the equator (zero latitude) and the Greenwich meridian plane (zero longitude). Parameters for point positioning within a coordinate system refer to the coordinate components of the system (either Cartesian or curvilinear). b. Geodetic Coordinates. Geodetic coordinate components consist of: latitude ( ), longitude (), ellipsoid height (h).

Geodetic latitude, longitude, and ellipsoid height define the position of a point on the surface of the Earth with respect to the reference ellipsoid. (1) Geodetic latitude ( ). The geodetic latitude of a point is the acute angular distance between the equatorial plane and the normal through the point on the ellipsoid measured in the meridian plane (Figure 4-2). Geodetic latitude is positive north of the equator and negative south of the equator. (2) Geodetic longitude (). The geodetic longitude is the angle measured counter-clockwise (east), in the equatorial plane, starting from the prime meridian (Greenwich meridian), to the meridian of the defined point (Figure 4-2). In the continental United States, longitude is commonly reported as a west longitude. To convert easterly to westerly referenced longitudes, the easterly longitude must be subtracted from 360 deg. East-West Longitude Conversion: (W) = [ 360 - (E) ] For example: (E) = 282 d 52 m 36.345 s E (W) = [ 360 d - 282 d 52 m 36.345 s E ] (W) = 77 d 07 m 23.655 s W (3) Ellipsoid Height (h). The ellipsoid height is the linear distance above the reference ellipsoid measured along the ellipsoidal normal to the point in question. The ellipsoid height is positive if the reference ellipsoid is below the topographic surface and negative if the ellipsoid is above the topographic surface.

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(4) Geoid Separation (N). The geoid separation (geoidal height) is the distance between the reference ellipsoid surface and the geoid surface measured along the ellipsoid normal. The geoid separation is positive if the geoid is above the ellipsoid and negative if the geoid is below the ellipsoid. (5) Orthometric Height (H). The orthometric height is the vertical distance of a point above or below the geoid.

IERS Conventional Terrestrial Pole

z

Point P on earths surface: Geocentric coordinates X-Y-Z Geographic coordinates (Lat-Long-height) - NAD 27 - NAD 83 z Local coordinates n-e-u

xMass center of earth

y

P f l

y xIERS WGS 84 Zero Meridian approx Greenwich

Figure 4-2. Coordinate reference frames

c. Datums. A datum is a coordinate surface used as reference for positioning control points. Both horizontal and vertical datums are commonly used in surveying and mapping to reference coordinates of points in a network. (1) Horizontal Datum. A horizontal datum is defined by specifying: the 2D geometric surface (plane, ellipsoid, sphere) used in coordinate, distance, and directional calculations; the initial reference point (origin); and a defined orientation, azimuth or bearing from the initial point. (a) Geodetic Datum. Five parameters are required to define an ellipsoid-based datum. The major semi-axis (a) and flattening (f) define the size and shape of the reference ellipsoid; the latitude and longitude of an initial point; and a defined azimuth from the initial point define its orientation with respect to the earth. The NAD 27 and NAD 83 systems are examples of horizontal geodetic datums. (b) Project Datum. A project datum is defined relative to local control and might not be directly referenced to a geodetic datum. Project datums are usually defined by a system with perpendicular axes, and with arbitrary coordinates for the initial point, and with one (principal) axis oriented toward true north. (d) NAD 27. NAD 27 is based on an adjustment of surveying measurements made between numerous control points using the Clarke 1866 reference ellipsoid. The origin and orientation of NAD 274-3

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is defined relative to a fixed triangulation station in Kansas (i.e., Meades Ranch). Azimuth orientation for NAD 27 is referenced to South, with the Greenwich Meridian for longitude origin. The distance reference units for NAD 27 are in US Survey Feet. NAD 27 was selected for North America. (e) NAD 83. NAD 83 is defined with respect to the Geodetic Reference System of 1980 (GRS 80) ellipsoid. GRS 80 is a geocentric reference ellipsoid. Azimuth orientation for NAD 83 is referenced to North with the Greenwich Meridian for longitude origin. The distance reference units for NAD 83 are in meters. (f) WGS 84. WGS 84 is defined with respect to the World Geodetic System of 1984 (WGS 84) ellipsoid. WGS 84 is a geocentric reference ellipsoid and is the reference system for the Global Positioning System (GPS). Azimuth orientation for WGS 84 is referenced to North, with the Greenwich Meridian for longitude origin. The distance reference units for WGS 84 are in meters. (2) Vertical Datum. A vertical datum is a reference system used for reporting elevations. Vertical datums are most commonly referenced to: Mean Sea Level (MSL), Mean Low Water (MLW), Mean Lower Low Water (MLLW), Mean High Water (MHW).

Mean Sea Level based elevations are used for most construction, photogrammetric, geodetic, and topographic surveys. MLLW elevations are used in referencing coastal navigation projects. MHW elevations are used in construction projects involving bridges over navigable waterways. (a) The vertical reference system formerly used by USACE was the National Geodetic Vertical Datum of 1929 (NGVD 29). The North American Vertical Datum of 1988 (NAVD 88) should be used by USACE for all vertical positioning surveying work. Transformations between NGVD 29 and NAVD 88 have been developed for general use--refer to Appendix C for details. 4-3. State Plane Coordinate System a. General. State Plane Coordinate Systems (SPCS) were developed by the National Geodetic Survey (NGS) to provide plane coordinates over a limited region of the earth's surface. To properly relate geodetic coordinates ( --h) of a point to a 2D plane coordinate representation (Northing, Easting), a conformal mapping projection must be used. Conformal projections have mathematical properties that preserve differentially small shapes and angular relationships as a result of the transformation between the ellipsoid and mapping plane. Map projections that are most commonly used for large regions are based on either a conic or a cylindrical mapping surface (Figure 4-3). The projection of choice is dependent on the north-south or east-west areal extent of the region. Areas with limited east-west dimensions and indefinite north-south extent use the Transverse Mercator (TM) type projection. Areas with limited north-south dimensions and indefinite east-west extent use the Lambert projection. The SPCS was designed to minimize the spatial distortion at a given point to approximately one part in ten thousand (1:10,000). To satisfy this criteria, the SPCS has been divided into zones that have a maximum width or height of approximately one hundred and fifty eight statute miles (158 miles). Therefore, each state may have several zones or may employ both the Lambert (conic) and Transverse Mercator (cylindrical) projections. The projection state plane coordinates must be referenced to a specific geodetic datum (i.e., the datum that the initial geodetic coordinates are referenced to must be known).

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ZImaginary Cone Imaginary Cylinder

Earth

Earth

LAMBERT CONFORMAL CONIC PROJECTION

TRANSVERSE MERCATOR PROJECTION

Figure 4-3. Common Map Projections

b. Transverse Mercator (TM). The Transverse Mercator projection uses a cylindrical surface to cover limited zones on either side of a central reference longitude. Its primary axis is rotated perpendicular to the symmetry axis of the reference ellipsoid. Thus, the TM projection surface intersects the ellipsoid along two lines equidistant from the designated central meridian longitude (Figure 4-4). Distortions in the TM projection increase predominantly in the east-west direction. The scale factor for the Transverse Mercator projection is unity where the cylinder intersects the ellipsoid. The scale factor is less than one between the lines of intersection, and greater than one outside the lines of intersection. The scale factor is the ratio of arc length on the projection to arc length on the ellipsoid. To compute the state plane coordinates of a point, the latitude and longitude of the point and the projection parameters for a particular TM zone or state must be known.

A

C

158 miles or less A C

Scale less than true

B

D

B D Scale greater than true Scales Exact

Scale greater than true

DEVELOPABLE CYLINDER

DEVELOPED CYLINDER

Figure 4-4. Transverse Mercator Projection

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c. Lambert Conformal Conic (LCC). The Lambert projection uses a conic surface to cover limited zones of latitude adjacent to two parallels of latitude. Its primary axis is coincident with the symmetry axis of the reference ellipsoid. Thus, the LCC projection intersects the ellipsoid along two standard parallels (Figure 4-5). Distortions in the LCC projection increase predominantly in the northsouth direction. The scale factor for the Lambert projection is equal to unity at each standard parallel and is less than one inside, and greater than one outside the standard parallels. The scale factor remains constant along the standard parallels.

Vertex

Projection Limits < 158 miles

R

Standard Parallels scale exact

Central Meridian

Figure 4-5. Lambert Projection

c. Scale units. State plane coordinates can be expressed in both feet and meters. State plane coordinates defined on the NAD 27 datum are published in feet. State plane coordinates defined on the NAD 83 datum are published in meters, however, state and federal agencies can request the NGS to provide coordinates in feet. If NAD 83 based state plane coordinates are defined in meters and the user intends to convert those values to feet, the proper meter-feet conversion factor must be used. Some states use the International survey foot rather than the US Survey foot in the conversion of feet to meters. International Survey Foot: 1 International Foot = 0.3048 meter (exact) US Survey Foot: 1 US Survey Foot = 1200 / 3937 meter (exact) 4-4. Universal Transverse Mercator Coordinate System a. General. Universal Transverse Mercator (UTM) coordinates are used in surveying and mapping when the size of the project extends through several state plane zones or projections. UTM coordinates are also utilized by the US Army, Air Force, and Navy for mapping, charting, and geodetic applications. The UTM projection differs from the TM projection in the scale at the central meridian,

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origin, and unit representation. The scale at the central meridian of the UTM projection is 0.9996. In the Northern Hemisphere, the northing coordinate has an origin of zero at the equator. In the Southern Hemisphere, the southing coordinate has an origin of ten million meters (10,000,000 m). The easting coordinate has an origin five hundred thousand meters (500,000 m) at the central meridian. The UTM system is divided into sixty (60) longitudinal zones. Each zone is six (6) degrees in width extending three (3) degrees on each side of the central meridian. The UTM system is applicable between latitudes eightyfour degrees north (84 d N) to eighty degrees south (80 d S). To compute the UTM coordinates of a point, the TM coordinates must be determined. The UTM northing or southing (N UTM, S UTM) coordinates are computed by multiplying the scale factor (0.9996) at the central meridian by the TM northing or southing (N TM, S TM) coordinate values. In the Southern Hemisphere, a ten million meter (10,000,000 m) offset must be added to account for the origin. The UTM eastings (E UTM) are derived by multiplying the TM eastings (E TM) by the scale factor of the central meridian (0.9996) and adding a five-hundred thousand meter (500,000 m) offset to account for the origin. UTM coordinates are always expressed in meters. UTM Northings, Southings, and Eastings Northern Hemisphere: N UTM = (0.9996) N TM Southern Hemisphere: S UTM = (0.9996) STM

+ 10,000,000 m

E UTM = (0.9996) E TM + 500,000 m The UTM zone (Z) can be calculated from the geodetic longitude of the point (converted to decimal degrees). In the continental United States, UTM zones range from ten (10) to nineteen (19). UTM Zone: Z = (180 + ) / 6 Z = (180 - ) / 6 where Z = UTM zone number If the computed zone value Z results in a decimal quantity, then the zone must be incremented by one whole zone number. Example of UTM Zone Calculation: = 77 d 08m 44.3456 s W Z = 17.14239 Z = 17 + 1 Z = 18 In the example above, Z is a decimal quantity, therefore, the zone equals seventeen (17) plus one (1).4-7

(east longitude) (west longitude)

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4-5. Datum Transformations a. General. Federal Geodetic Control Subcommittee (FGCS) members, which includes USACE, have adopted NAD 83 as the standard horizontal datum for surveying and mapping activities performed or financed by the Federal government. To the extent practicable, legally allowable, and feasible, USACE should use NAD 83 in its surveying and mapping activities. Transformations between NAD 27 coordinates and NAD 83 coordinates are generally obtained using the CORPS Convert (i.e., CORPSCON) software package or other North American Datum Conversion (i.e., NADCON) based programs. b. Conversion techniques. USACE survey control published in the NGS control point database has been already converted to NAD 83 values. However, most USACE survey control was not originally in the NGS database and was not included in the NGS readjustment and redefinition of the national geodetic network. Therefore, USACE will have to convert this control to NAD 83. Coordinate conversion methods considered applicable to USACE projects are discussed below. (1) Resurvey from NAD 83 Control. A new survey using NGS published NAD 83 control could be performed over the entire project. This could be either a newly authorized project or one undergoing major renovation or maintenance. Resurvey of an existing project must tie into all monumented points. Although this is not a datum transformation technique, and would not normally be economically justified unless major renovation work is being performed, it can be used if existing NAD 27 control is of low density or accuracy. (2) Readjustment of Survey. If the original project control survey was connected to NGS control stations, the survey may be readjusted using the NAD 83 coordinates instead of the NAD 27 coordinates originally used. This method involves locating the original field notes and observations, and completely readjusting the survey and fixing the published NAD 83 control coordinates. (3) Mathematical Transformations. Since neither of the above methods can be economically justified on most USACE projects, mathematical approximation techniques for transforming project control data to NAD 83 have been developed. These methods yield results which are normally within 1 foot of the actual values and the distribution of errors are typically consistent within a local project area. Since these coordinate transformation techniques involve approximations, they should be used with caution when real property demarcation points and precise surveying projects are involved. When mathematical transformations are employed they should be adequately noted so that users will be aware of the conversion method. 4-6. Horizontal Datum Transformations a. General. Coordinate transformations from one geodetic reference system to another can be most practically made by using either a local seven-parameter transformation, or by interpolation of datum shift values across a given region. b. Seven parameter transformations. For worldwide (OCONUS) and local datum transformations, the procedures referenced in USATEC SR-7 1996, "Handbook for Transformation of Datums, Projections, Grids and Common Coordinate Systems" should be consulted. This document contains references for making generalized datum shifts and working with a variety of commonly used map projections.

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c. Grid-shift transformations. Current methods for interpolation of datum shift values use the difference between known coordinates of common points from both the NAD 27 and NAD 83 adjustments to model a best-fit shift in the regions surrounding common points. A grid of approximate datum shift values is established based on the computed shift values at common points in the geodetic network. The datum shift values of an unknown point within a given grid square are interpolated along each axis to compute an approximate shift value between NAD 27 and NAD 83. Any point that has been converted by such a transformation method, should be considered as having only approximate NAD 83 coordinates. d. NADCON/CORPSCON. NGS developed the transformation program NADCON, which yields consistent NAD 27 to NAD 83 coordinate transformation results over a regional area. This technique is based on the above grid-shift interpolation approximation. NADCON was reconfigured into a more comprehensive program called CORPSCON. This software converts between: NAD 27 UTM 27 GEOID96 NAD 83 UTM 83 HARN SPCS 27 NGVD 29 SPCS 83 NAVD 88

Technical documentation and operating instructions for CORPSCON are listed in Appendix B. Since the overall CORPSCON datum shift (from point to point) varies throughout North America, the amount of datum shift across a local project is also not constant. The variation can be as much as 0.1 foot per mile. Some typical NAD 27 to NAD 83 based coordinate shift variations that can be expected over a 10,000 foot section of a project are shown below: Project Area Baltimore, MD Los Angeles, CA Mississippi Gulf Coast Mississippi River (IL) New Orleans, LA Norfolk, VA San Francisco, CA Savannah, GA Seattle, WA SPCS Reference 1900 0405 2301 1202 1702 4502 0402 1001 4601 Per 10,000 feet 0.16 ft 0.15 ft 0.08 ft 0.12 ft 0.22 ft 0.08 ft 0.12 ft 0.12 ft 0.10 ft

Such local scale changes will cause project alignment data to distort by unequal amounts. Thus, a 10,000foot tangent on NAD 27 project coordinates could end up as 9,999.91 feet after mathematical transformation to NAD 83 coordinates. Although such differences may not be appear significant from a lower-order construction survey standpoint, the potential for such errors must be recognized. Therefore, the transformations will not only significantly change absolute coordinates on a project, the datum transformation process will slightly modify the project's design dimensions and/or construction orientation and scale. On a navigation project, for example, an 800.00 foot wide channel could vary from 799.98 to 800.04 feet along its reach, and also affect grid azimuths. Moreover, if the local SPCS 83 grid was further modified, then even larger dimension changes can result. Correcting for distortions may require recomputation of coordinates after conversion to ensure original project dimensions and alignment data remain intact. This is particularly important for property and boundary surveys. A less accurate alternative is to compute a fixed shift to be applied to all data points over a limited area. Determining the maximum area over which such a fixed shift can be applied is important. Computing a fixed conversion factor with CORPSCON can be made to within 1 foot. Typically, this fixed conversion would be computed at the center of a sheet or at the center of a project and the conversions in X and Y from NAD 27 to NAD 83 and from SPCS 27 to SPCS 83 indicated by notes on the sheets or data sets. Since the4-9

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conversion is not constant over a given area, the fixed conversion amounts must be explained in the note. The magnitude of the conversion factor change across a sheet is a function of location and the drawing scale. Whether the magnitude of the distortion is significant depends on the nature of the project. For example, a 0.5-foot variation on an offshore navigation project may be acceptable for converting depth sounding locations, whereas a 0.1 foot change may be intolerable for construction layout on an installation. In any event, the magnitude of this gradient should be computed by CORPSCON at each end (or corners) of a sheet or project. If the conversion factor variation exceeds the allowable tolerances, then a fixed conversion factor should not be used. Two examples of using Fixed Conversion Factors follow: (1) Example 1. Assume we have a 1" = 40' scale site plan map on existing SPCS 27 (VA South Zone 4502). Using CORPSCON, convert existing SPCS 27 coordinates at the sheet center and corners to SPCS 83 (US Survey Foot), and compare SPCS 83-27 differences. Center of Sheet NW Corner NE Corner SE Corner SW Corner SPCS 83 N 3,527,095.554 E 11,921,022.711 N 3,527,595.553 E 11,920,522.693 N 3,527,595.556 E 11,921,522.691 N 3,526,595.535 E 11,921,522.702 N 3,526,595.535 E 11,920,522.704 SPCS 27 Y 246,200.000 X 2,438,025.000 Y 246,700.000 X 2,437,525.000 Y 246,700.000 X 2,438,525.000 Y 245,700.000 X 2,438,525.000 Y 245,700.000 X 2,437,525.000 SPCS 83 - SPCS 27 dY = 3,280,895.554 dX = 9,482,997.711 dY = 3,280,895.553 dX = 9,482,997.693 dY = 3,280,895.556 dX = 9,482,997.691 dY = 3,280,895.535 dX = 9,482,997.702 dY = 3,280,895.535 dX = 9,482,997.704

Since coordinate differences do not exceed 0.03 feet in either the X or Y direction, the computed SPCS 83-27 coordinate differences at the center of the sheet may be used as a fixed conversion factor to be applied to all existing SPCS 27 coordinates on this drawing. (2) Example 2. Assuming a 1" = 1,000' base map is prepared of the same general area, a standard drawing will cover some 30,000 feet in an east-west direction. Computing SPCS 83-27 differences along this alignment yields the following: West End East End SPCS 83 N 3,527,095.554 E 11,921,022.711 N 3,527,095.364 E 11,951,022.104 SPCS 27 Y 246,200.000 X 2,438,025.000 Y 246,200.000 X 2,468,025.000 SPCS 83 - SPCS 27 dY = 3,280,895.554 dX = 9,482,997.711 dY = 3,280,895.364 dX = 9,482,997.104

The conversion factor gradient across this sheet is about 0.2 feet in Y and 0.6 feet in X. Such small changes are not significant at the plot scale of 1" = 1,000'; however, for referencing basic design or construction control, applying a fixed shift across an area of this size is not recommended -- individual points should be transformed separately. If this 30,000-foot distance were a navigation project, then a fixed conversion factor computed at the center of the sheet would suffice for all bathymetric features. Caution should be exercised when converting portions of projects or military installations or projects that are adjacent to other projects that may not be converted. If the same monumented control points are used4-10

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for several projects or parts of the same project, different datums for the two projects or parts thereof could lead to surveying and mapping errors, misalignment at the junctions and layout problems during construction. e. Dual grids ticks. Depicting both NAD 27 and NAD 83 grid ticks and coordinate systems on maps and drawings should be avoided where possible. This is often confusing and can increase the chance for errors during design and construction. However, where use of dual grid ticks and coordinate systems is unavoidable, only secondary grid ticks in the margins will be permitted. f. Global Positioning System (GPS). GPS surveying techniques and computations are based on WGS 84 coordinates, which are highly consistent with NAD 83. Differential (static) GPS surveying techniques are accurate for high order control over very large distances. If GPS is used to set new control points referenced to higher order control many miles from the project, inconsistent data may result at the project site. If the new control is near older control points that have been converted to NAD 83, two slightly different network solutions can result, even though both have NAD 83 coordinates. In order to avoid this situation, locate the GPS base stations on the control in the project area, (i.e., don't transfer it in from outside the area). Use the CORPSCON program to convert the old control from NAD 27 to NAD 83 and use these NAD 83 values to initiate the GPS survey. This allows GPS to produce coordinates that are both referenced to NAD 83 and consistent with the old control. g. Local project datums . Local project datums that are not referenced to NAD 27 cannot be mathematically converted to NAD 83 with CORPSCON. Field surveys connecting them to other stations that are referenced to NAD 83 are required. 4-7. Horizontal Transition Plan a. General. Not all maps, engineering site drawings, documents and associated products containing coordinate information will require conversion to NAD 83. To insure an orderly and timely transition to NAD 83 is achieved for the appropriate products, the following general guidelines should be followed: (1) Initial surveys. All initial surveys should be referenced to NAD 83. (2) Active projects. Active projects where maps, site drawings or coordinate information are provided to non-USACE users (e.g., NOAA, USCG, FEMA and others in the public and private sector) coordinates should be converted to NAD 83 the next time the project is surveyed or maps or site drawings are updated for other reasons. (3) Inactive projects. For inactive projects or active projects where maps, site drawings or coordinate information are not normally provided to non-USACE users, conversion to NAD 83 is optional. (4) Datum notes. Whenever maps, site drawings or coordinate information (regardless of type) are provided to non-USACE users, it should contain a datum note, such as the following: THE COORDINATES SHOWN ARE REFERENCED TO NAD *[27/83] AND ARE IN FEET BASED ON THE SPCS *[27/83] *[STATE, ZONE]. DIFFERENCES BETWEEN NAD 27 AND NAD 83 AT THE CENTER OF THE *[SHEET/DATASET] ARE *[dLat, dLon, dX, dY]. DATUM CONVERSION WAS PERFORMED USING THE COMPUTER PROGRAM "CORPSCON." METRIC CONVERSIONS WERE

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BASED ON THE *[US SURVEY FOOT = 1200/3937 METER] [INTERNATIONAL FOOT = 30.48/100 METER]. b. Levels of effort. For maps and site drawings the conversion process entails one of three levels of effort: (1) conversion of coordinates of all mapped details to NAD 83, and redrawing the map, (2) replace the existing map grid with a NAD 83 grid, (3) simply adding a datum note. For surveyed points, control stations, alignment, and other coordinated information, conversion must be made through either a mathematical transformation or through readjustment of survey observations. c. Detailed instructions. (1) Initial surveys on Civil Works projects. The project control should be established on NAD 83 relative to NGS National Geodetic Reference System (NGRS) using conventional or GPS surveying procedures. The local SPCS 83 grid should be used on all maps and site drawings. All planning and design activities should then be based on the SPCS 83 grid. This includes supplemental site plan mapping, core borings, project design and alignment, construction layout and payment surveys, and applicable boundary or property surveys. All maps and site drawings shall contain datum notes. If the local sponsor requires the use of NAD 27 for continuity with other projects that have not yet converted to NAD 83, conversion to NAD 27 could be performed using the CORPSCON transformation techniques described in Appendix B. (2) Active Civil Works Operations and Maintenance projects undergoing maintenance or repair. These projects should be converted to NAD 83 during the next maintenance or repair cycle in the same manner as for newly initiated civil works projects. However, if resources are not available for this level of effort, either redraw the grids or add the necessary datum notes. Plans should be made for the full conversion during a later maintenance or repair cycle when resources can be made available. (3) Military Construction and master planning projects. All installations and master planning projects should remain on NAD 27 or the current local datum until a thoroughly coordinated effort can be arranged with the MACOM and installation. An entire installation's control network should be transformed simultaneously to avoid different datums on the same installation. The respective MACOMs are responsible for this decision. However, military operations may require NAD 83, including SPCS 83 or UTM metric grid systems. If so, these shall be performed separate from facility engineering support. A dual grid system may be required for such operational applications when there is overlap with normal facilities engineering functions. Coordinate transformations throughout an installation can be computed using the procedure described in Appendix B. Care must be taken when using transformations from NAD 27 with new control set using GPS methods from points remote from the installation. Installation boundary surveys should adhere to those outlined under real estate surveys listed below. (4) Real Estate. Surveys, maps and plats prepared in support of civil works and military real estate activities should conform as much as possible to state requirements. Since most states have adopted NAD 83, most new boundary and property surveys should be based on NAD 83. The local authorities should be contacted before conducting boundary and property surveys to ascertain their policies. It should be noted that several states have adopted the International Foot for their standard conversion from meters to feet. In order to avoid dual coordinates on USACE survey control points that4-12

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have multiple uses, all control should be based on the US Survey Foot, including control for boundary and property surveys. In states where the International foot is the only accepted standard for boundary and property surveys, conversion of these points to NAD 83 should be based on the International foot, while the control remains based on the US Survey foot. (5) Regulatory functions. Surveys, maps and site drawings prepared in support of regulatory functions should begin to be referenced to NAD 83 unless there is some compelling reason to remain on NAD 27 or locally used datum. Conversion of existing surveys, maps and drawing to NAD 83 is not necessary. Existing surveys, maps and drawings need only have the datum note added before distribution to non-USACE users. The requirements of local, state and other Federal permitting agencies should be ascertained before site specific conversions are undertaken. If states require conversions based on the International foot, the same procedures as described above for Real Estate surveys should be followed. (6) Other existing projects. Other existing projects, e.g., beach nourishment, submerged offshore disposal areas, historical preservation projects, etc., need not be converted to NAD 83. However, existing surveys, maps and drawings should have the datum note added before distribution to non-USACE users. (7) Work for others. Existing projects for other agencies will remain on NAD 27 or the current local datum until a thoroughly coordinated effort can be arranged with the sponsoring agency. The decision to convert rests with the sponsoring agency. However, existing surveys, maps and drawings should have the datum note added before distribution to non-USACE users. If sponsoring agencies do not indicate a preference for new projects, NAD 83 should be used. The same procedures as described above for Initial Surveys on Civil Works Projects should be followed. 4-8. Vertical Datums a. General. A vertical datum is the surface to which elevations or depths are referred to or referenced. There are many vertical datums used within CONUS. The surveyor should be aware of the vertical control datum being used and its practicability to meet project requirements. Further technical details on vertical datums are in Appendix C, Development and Implementation of NAVD 88. b. NGVD 29. NGVD 29 was established by the United States Coast and Geodetic Survey (USC&GS) 1929 General Adjustment by constraining the combined US and Canadian First Order leveling nets to conform to Mean Sea Level (MSL). It was determined at 26 long term tidal gage stations that were spaced along the east and west coast of North American and along the Gulf of Mexico, with 21 stations in the US and 5 stations in Canada. NGVD 29 was originally named the Mean Sea Level Datum of 1929. It was known at the time that the MSL determinations at the tide gages would not define a single equipotential surface because of the variation of ocean currents, prevailing winds, barometric pressures and other physical causes. The name of the datum was changed from the Mean Sea Level Datum to the NGVD 29 in 1973 to eliminate the reference to sea level in the title. This was a change in name only; the definition of the datum established in 1929 was not changed. Since NGVD 29 was established, it has become obvious that the geoid based upon local mean tidal observations would change with each measurement cycle. Estimating the geoid based upon the constantly changing tides does not provide a stable estimate of the shape of the geoid. c. NAVD 88. NAVD 88 is the international vertical datum adopted for use in Canada, the United States and Mexico. NAVD 88 is based on gravity measurements made at observation points in the network and only one datum point, at Pointe-au-Pere/Rimouski, Quebec, Canada, is used. The vertical reference surface is therefore defined by the surface on which the gravity values are equal to the control point value. The result of this adjustment is newly published NAVD 88 elevation values for benchmarks (BM) in the NGS inventory. Most Third Order benchmarks, including those of other Federal, state and4-13

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local government agencies, were not included in the NAVD 88 adjustment. The Federal Geodetic Control Subcommittee (FGCS) of the Federal Geographic Data Committee (FGDC) has affirmed that NAVD 88 shall be the official vertical reference datum for the US. The FGDC has prescribed that all surveying and mapping activities performed or financed by the Federal Government make every effort to begin an orderly transition to NAVD 88, where practicable and feasible. Procedures for performing this transition are outlined in Appendix E. d. Mean Sea Level datums. Some vertical datums are referenced to mean seal level. Such datums typically are maintained locally or within a specific project area. The theoretical basis for these datums is local mean sea level. Local MSL is a vertical datum based on observations from one or more tidal gaging stations. NGVD 29 was based upon the assumption that local MSL at 21 tidal stations in the US and 5 tidal stations in Canada equaled 0.0000 foot on NGVD 29. The value of MSL as measured over the Metonic cycle of 19 years shows that this assumption is not valid and that MSL varies from station to station. e. Lake and tidal datums. Some vertical datums are referenced to tidal waters or lake levels. An example of a lake level used as a vertical datum is the International Great Lakes Datum of 1955 (IGLD 55), maintained and used for vertical control in the Great Lakes region of CONUS. These datums undergo periodic adjustment. For example, the IGLD 55 was adjusted in 1985 to produce IGLD 85. IGLD 85 has been directly referenced to NAVD 88 and originates at the same point as NAVD 88. Tidal datums typically are defined by the phase of the tide and usually are described as mean high water, mean low water, and mean lower low water. For further information on these and other tidal datum related terms, the reader is advised to refer to Appendix D (Requirements and Procedures for Referencing Coastal Navigation Projects to Mean Lower Low Water (MLLW) Datum) and EM 1110-2-1003 (Hydrographic Surveying). f. Other vertical datums. Other areas may maintain and employ specialized vertical datums. For instance, vertical datums maintained in Alaska, Puerto Rico, Hawaii, the Virgin Islands, Guam, and other islands and project areas. Specifications and other information for these particular vertical datums can be obtained from the particular FOA responsible for survey related activities in these areas, or the National Ocean Service (NOS). 4-9. Vertical Datum Transformations a. General. There are several reasons for USACE commands to convert to NAVD 88. (1) Differential leveling surveys will close better. (2) NAVD 88 will provide a reference to estimate GPS derived orthometric heights. (3) NAVD 88 Height values will be available in convenient form from the NGS database. (4) Federal surveying and mapping agencies will stop publishing on NGVD 29. (5) NAVD 88 is recommended by ACSM and FGCS. (6) Surveys performed for the Federal government will require use of NAVD 88. The conversion process entails one of two levels of effort: (1) conversion of all elevations to NAVD 88 and redrawing the map,4-14

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(2) adding a datum note based on an approximate conversion. b. VERTCON. VERTCON is a software program developed by NGS that converts elevation data from NGVD 29 to NAVD 88. Although the VERTCON software has been fully incorporated into the software application package CORPSCON, it will be referred to below as a separate program. VERTCON uses benchmark heights to model the shift between NGVD 29 and NAVD 88 that is applicable to a given area. In general, it is only sufficiently accurate to meet small-scale mapping requirements. VERTCON should not be used for converting benchmark elevations used for site plan design or construction applications. Users input the latitude and longitude for a point and the vertical datum shift between NGVD 29 and NAVD 88 is reported. The root-mean-square (RMS) error of NGVD 29 to NAVD 88 conversion, when compared to the stations used to create the conversion model, is 1 cm; with an estimated maximum error of 2.5 cm. Depending on network design and terrain relief, larger differences (e.g., 5 to 50 cm) may occur the further a bench mark is located from the control points used to establish the model coefficients. For this reason, VERTCON should only be used for approximate conversions where these potential errors are not critical. c. Datum note. Whenever maps, site drawings or spatial elevation data are provided to nonUSACE users, they should contain a datum note that provides, at minimum, the following information: The elevations shown are referenced to the *[NGVD 29] [NAVD 88] and are in *[feet] [meters]. Differences between NGVD 29 and NAVD 88 at the center of the project sheet/data set are shown on the diagram below. Datum conversion was performed using the *[program VERTCON] [direct leveling connections with published NGS benchmarks] [other]. Metric conversions are based on *[US Survey Foot = 1200/3937 meters] [International Survey Foot = 0.3048 meters]. 4-10. Vertical Transition Plan a. General. A change in the accepted vertical datum will affect USACE engineering, construction, planning, and surveying activities. The cost of conversion could be substantial at the onset. There is a potential for errors in conversions inadvertently occurring. The effects of the vertical datum change can be minimized if the change is gradually applied over time; being applied to future projects and efforts, rather than concentrated on changing already published products. In order to insure an orderly and timely transition to NAVD 88 is achieved for the appropriate products, the following general guidelines should be followed. b. Conversion criteria. Maps, engineering site drawings, documents and associated spatial data products containing elevation data may require conversion to NAVD 88. Specific requirements for conversion will, in large part, be based on local usage -- that of the local sponsor, installation, etc. Where applicable and appropriate, this conversion should be recommended to local interests. c. Newly authorized construction projects. Generally, initial surveys of newly authorized projects should be referenced to NAVD 88. In addition to design/construction, this would include widearea master plan mapping work. The project control should be referenced to NAVD 88 using conventional or GPS surveying techniques. All planning and design activities should be based upon NAVD 88. All maps and site drawings shall contain datum notes as described below. If the sponsor/installation requires the use of NGVD 29 or some other local vertical reference datum for continuity, the relationship between NGVD 29 and NAVD 88 shall be clearly noted on all maps, engineering site drawings, documents and associated products.

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d. Active projects. On active projects where maps, site drawings, or elevation data are provided to non-USACE users, the conversion to NAVD 88 should be performed. This conversion to NAVD 88 may be performed the next time the project is surveyed or when the maps/site drawings are updated for other reasons. Civil works projects may be converted to NAV